Compound semiconductor devices



Aug. 16, 1966 J, McCALDlN 3,266,952

COMPOUND SEMICONDUCTOR DEVICES Filed March 25. 1965 Fig. 4 4

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AT TOR NEY.

United States Patent 3,266,952 COMPOUND SEMICONDUCTOR DEVICES James O.McCaldin, Los Angeles, Calif., assignor to Hughes Aircraft Company,Culver City, Calif., a corporation of Delaware Filed Mar. 25, 1965, Ser.No. 442,667 3 Claims. (Cl. 148-33) This application is acontinuation-in-part of application Serial No. 42,878 filed July 14,1960, now abandoneld.

This invention relates to compound semiconductor devices, and moreparticularly to devices commonly known as IIIV (Periodic Table Groups)compounded semicondu'ctor containing impurity materials of Group IVelements such as silicon and tin.

It has been reported that compound semiconductor materials or the III-Vtype containing quantities of column IV material as impurity, such asgermanium, tin and silicon have a characteristic conductivity type, withP- type or N-type, and accordingly for such compounds the column IVmaterials have been known as dopants. No mechanism for such doping hasexplained the doping characteristics so produced, and suchcharacteristics are at best erratic and unpredictable as to apparentdoping concentration and, at times, doping type.

The production of conductivity-determining types in compoundsemiconductors, such as gallium-arsenide, does not follow the same rulesand procedures as for column IV (of the periodic table) semiconductormaterials such as silicon and germanium. Doping of such III-V compoundsemiconductor crystals with column II or column VI element impurities iscommon practice today, as noted by Edmond, Proc. Phys. Rev., vol. 73,pt. 4, 622-7 (April 1959). This requires a double doping procedure,generally in independent steps, and presents many problems of unwantedchemical reactions, unwanted impurities, and complex material handling.This also makes it quite diflicult to precisely control conditions toproduce thin base transistor devices.

The doping of III-V compound semiconductors with elements [from columnIV has been studied in special cases, and has been reported in theliterature as producing N-type conductivity in most cases, the IVelement having a low doping efficiency. This has been interpreted tomean that more of the impurity atoms are located on the column IIIelement sublattice than on the column V element sublattice of thesemiconductor crystal. When III-V semiconductor compounds are producedin a standard procedure, their conductivity type is predictable, atleast on the basis of prior experience.

According to the present invention, it is believed that the vacancies inthe III and V element sublattices may be readjusted, and the relativepositions in the sub'lattices of the III and V elements occupied by theIV element adjusted, by exposure of a column IV element doped III-Vsemiconductor crystal (whose constituent elements have substantiallydilierent vapor pressures) to a controlled temperature, time, andatmosphere pressure of the more volatile of the III and V elements. Inevery III-V compound presently under practical consideration, the V elernent is considerably the more volatile.

This invention provides compound semiconductor devices having PNjunctions therein as a result of the apparent doping characteristics ofcolumn IV element impurities in each of the P and N-type regions. By wayof example, this invention relates to gallium-arsenide semiconductordevices containing germanium as an impurity in both the P-type andN-type regions forming therein a PN junction. Other characteristics andadvantages of this invention will be apparent from the balance of thisdisclosure and the preferred embodiment of the invention illustratedtherein and in the accompanying drawing forming a part thereof, herein:

FIG. 1 is an incomplete three-element phase diagram;

FIG. 2 is a diagram showing conductivity type as a function of arsenicpartial pressure and germanium concentration at equilibrium in agalliurnaarsenide semiconductor crystal;

FIG. 3 is a sectional view of a diode made according to this invention;

FIG. 4 is a sectional view of a transistor according to this invention.

This invention is illustrated for the III-V semiconductorgallium-arsenide, doped with the column IV impurity germanium.

Semiconductor crystals of gallium-arsenide have a nominal, or chemicallydeterminable, composition as shown in FIG. 1, [a partial phase diagramfor the system gallitun, arsenic, and germanium. Line 21 representssemiconductors at 0.5 (50 atomic percent) gallium, 0.5 arsenic, and O toover 1% germanium compositions. The gallium-arsenide semiconductorcrystals fall on the 50% line, and the germanium dopant may be up to thesolubility limit, which is at least 1%, although 0.01 to 1.1% ispresently preferred. The curves 11 and 12 are schematic, and may notrepresent the actual shape of the true curves for the physical data.

In the system gallium-arsenic-genmanium, the arsenic is relativelyvolatile with respect to gallium (and germanium). Although theproportions, for chemical purposes, of gallium and arsenic in thesemiconductor crystal do not appreciably change with a change in arsenicpres sure over a crystal, it has been found that, by apparentin-difiusion or out-diffusion of arsenic due to controlled vaporpressure and temperature, the conductivity type or the surface-adjacentcrystal region may be changed. It is believed that the proportion oflattice vacancies is shifted by adding :or removing arsenic atoms, andthat germanium atoms then tend to redistribute between the sublat-ticesof gallium and arsenic, thus changing the conductivity type of thecrystal. Higher pressures P of arsenic reduce the concentration ofvacancies V in the arsenic sublattice of the crystal, and by a transferreaction which may be simplified as where Ge and Ge are the germaniumatoms in the respective gallium and arsenic sublattices.

A mass action relationship for equilibrium between vacancies andgermanium atoms may be written:

Ge VAs where N the concentration of vacancies in the arsenic lattice,depends upon the pressure of arsenic in the system N P=K (for amonatomic gas), so Equation 2 above may be written in terms of gaspressures. At equilibrium, the gas pressures of As and Ga varyinversely, and

Ge a As As shown in FIG 2, the equilibrium conductivity type of agermanium doped gallium-arsenide semiconductor crystal changes on line12 with the arsenic pressure. For this system, crystals prepared from agallium-rich melt 4 pressure less than 0.1 atmosphere. The reconversionto P-type is preferably at a lower temperature to provide better controlof diffusion depth. It will be appreciated that NPN structures may beproduced from originally have an effective P for arsenic of less than 1atmosphere, 5 N-type crystals; and PN diodes from originally N-type andare P-type, hence P is believed to be less than 1 crystals byout-diffusion under low arsenic vapor presatmosphere, although theprecise pressure is not known. sure. Different production techniquesvary the effective atmos- The process of PN junction formation may beapplied phere pressures, and a horizontal zone melting technique to avariety of III-V compounds. Commercially, or has been used to produceN-type material under 1 atmoschemically, pure gallium-ars'enidesemiconductor matephere arsenic vapor. rial is believed to containsufficient silicon, a column IV While FIG. 2 assumes a nominal galliumpressure has element, to accommodate the process herein described, nosubstantial effect on the system, due to the kinetics of and achemically pure crystal of gallium-arsenlde was the reactions, lowgallium atmosphere pressure does have type changed by the pressureadjustment process herein a slow, surface effect. This is known as asurface erodescribed. sion of the crystal, and it is preferablysuppressed by use Normal semiconductor production procedures for III-Vof an inert gas blanket of 1 atmosphere argon with the compounds varyfrom compound to compound, primarily arsenic vapor. in the crystalpulling temperature and the ambient pres- While accurate prediction ofconductivity type and sure Of the V element atmosphere used. The Velement other impurity connected properties is not alwayspossiatmosphere pressure used for nor l cry l g g, ble, it is arelatively simple matter to measure such propcalled herein the normalcry growing Pressure, erties, then to set conditions to change theconductivity where attainable, the pressure of the V element which typeand thus to produce a PN junction. Dashed line 11 under stoichioimetricconditions is in equilibrium with a represents equilibrium limit to theP and N regions of the 1 1 iquid Iuti Il 0f the III and V elem nts.semiconductor crystal structure, and dashed line 12 repre- Th n0rrnalcrystal growing temperature will be the sents th intrin i values, Theprecise lo tio f th freezing temperature for the semiconductor materialat lines is not exactly known. the ambient pressure used, and will ofcourse vary for a A galiliurn-arsenide crystal having 1% germanium wasgiven semi-conductor material as the pressure used varies produced bythe Czochralski method of crystal drawing m t e t iChi' m tIiC normalpressure. under 1 atmosphere arsenic vapor pressure. Thus the In a givennormal crystal Production Process, the crystal fell schematically atpoint 22 in FIG. 2, in a P- ductivity yp Will be affected y changes inthe IV typ'e region of the diagram. A slice of the crystal was ment usedas all p y, but will Ordinarily be uniform subjected to 70 hours at1100" C, and at 5 t o h for a given impurity through a range ofconcentrations. arsenic pressure. The surface of the crystal was con-Thus, in the follOWing Table Normal GTOWing Presvented to N d h PN j iwas from 30 isures (absolute) and Normal Growing Temperatures crons to70 microns below the exposed crystal surface. are given Various III-VSemiconductor Compounds, Thus in FIG. 2, the surface characteristicmoved on line far as Presently knOWn, and Normal Conductivity 23 topoint 24 in the N region during the above high ar- Types so producedwith various impurities of column IV.

TABLE I Normal Normal III-V Normal Growing Pressure Growing Column IVConduc- Compound Tempera- Impurity tivity ture, C. Type In As 0.3Atmosphere of As 936 Si N Go N Sn N In Sb Below 1 micron Hg of Sb 530 SiGe P Sn N In P 15 to G0 Atmospheres of 1, 060 Si 3 Ge N Sa N Ga As 0.9Atmosphere As 1, 240 Si N Go N Sn N Ga Sb Less than 250 microns Hg 702Si P (0.0003 Atm.). Ge P Sn P Ga P Above 10 Atm 1, 450 Si N senicpressure treatment. Capacity vs. reverse bias meas- From the above TableI, taken with the discussion of urements indicated linear grading forthe doping, mm FIG. 2, it should be readily apparent that agalliumfirmingadifiusion type process. antimonide semiconductor havingsilicon, germanium FIG. 3 shows a diode made from a gallium-arsenide ortin as a column IV impurity, will ordinarily be P-type crystal slice ofP-type, converted to N-type at the surconductivity as produced. It willbe subject to converfiace as above described. A crystal 31 having P andN sion to N-type by subjection to a diffusion treatment in regions and ajunction 32 is bonded to a tantalum lead 33 an ambient antimony vaporatmosphere considerably in by a gold bond 34. excess of 0.0003atmosphere and at a temperature suf- FIG. 4 shows a similar transistorstructure prior to ficiently below 702 C. to maintain the semiconductoretching a surface area for base lead attachment. A crystal structure,usually 100 to 300 C. below the freezg alliuim-arsenide crystal 41having PN junctions 42, 43 is bonded to a tantalum lead 46 by a goldbond 45.

To produce the transistor structure, a P-type crystal is subjected firstto very high arsenic vapor pressure such as 5 atmospheres, thensubsequently to a very low vapor ing temperature. The depth of the Nregion formed by this treatment will, of course depend upon thetemperature selected and the time of treatment.

Similarly, a semiconductor material of indium-arsenide having as acolumn IV impurity silicon, germanium or tin,

should be subject to conversion to P-type at an ambient arsenicatmosphere pressure substantially less than 0.3 atmosphere of arsenicand at a suitable temperature.

The pressure and temperature selected to convert P- type to N-typeshould not be so high as to change the semiconductor material to aliquid phase; and similarly, the pressure and temperature selected toconvert N-type to P-type to a vapor phase. In other words, discretionmust be used to avoid changing the semiconductor crystal phase beforethe conductivity type is changed.

It may be noted that the principles herein taught apply to other III-Vsemiconductors, such as aluminum-phosphide or aluminum-arsenide,although they are not attractive presently as semiconductor materialsbecause of their hygroscopic properties; and indium-antimonide with tinas a predominant IV element impurity, which is unattractive because suchloW pressures would be required.

The doping characteristics of the devices according to this inventionmay be of lower concentration than desired in some applications. In suchcases conventional doping materials for compound semiconductors such asthe column II elements and the column VI elements may be used asadditional dopants in the respective N and P regions. Such additionaldopants may be of such value in the contact area to which leads areattached to avoid any disturbance of the conductivity type of suchregion during lead attachment. For example, zinc or cadmium from columnII or sulphur or selenium from column VI may be used in the normaldoping fashion to produce high doping concentration in a device adjacentthe leads. Such elements may be present in the alloy material during thelead attachment bonding process to prevent a reversion of conductivitytype to an undesired type of concentration. Similarly such dopingimpurity may be diffused or alloyed to the exposed face of a diodecrystal prior to lead attachment. Opposite conductivity type dopantmaterials may be applied to opposed crystal surfaces in crystals alreadyhaving a PN junction resulting from the effect of the column IV elementas heretofore explained.

What is claimed is:

1. A III-V semiconductor crystal PN junction device 'having asubstantially uniform impurity concentration of a column IV element,with a first region having a sufiicient excess of said impurity in theIII element sublattice to exhibit N-type conductivity and having asecond region With a sufiicient excess of said impurity in the V elementsublattice to exhibit P-type conductivity, said crystal beingessentially free from conductivity type determining impurities ofcolumns II and VII in the region of the P-N junction.

2. A device according to claim 1 wherein said III-V crystal is acompound of the class consisting of:

InAs GaAs InSb GaSb InP GaP 3. A device according to claim 1 whereinsaid column IV element is .an element of the class consisting ofsilicon, germanium and tin.

References Cited by the Examiner UNITED STATES PATENTS 3/ 1960Gremmelmaier et al.

OTHER REFERENCES DAVID L. RECK, Primary Examiner.

BENJAMIN HENKIN, Examiner.

C. N. LOVELL, Assistant Examiner.

1. A III-V SEMICONDUCTOR CRYSTAL PN JUNCTION DEVICE HAVING ASUBSTANTIALLY UNIFORM IMPURIY CONCENTRATION OF A COLUMN IV ELEMENT, WITHA FIRST REGION HAVING A SUFFICIENT EXCESS OF SAID IMPURITY IN THE IIIELEMENT SUBLATTICE TO EXHIBIT N-TYPE CONDUCTIVITY AND HAVING A SECONDREGION WITH A SUFFICIENT EXCESS OF SAID IMPURITY IN THE V ELEMENTSUBLATTICE TO EXHIBIT P-TYPE CONDUCTIVITY, SAID CRYSTALS BEINGESSENTIALLY FREE FROM CONDUCTIVITY TYPE DETERMINING IMPURITIES OFCOLUMNS II AND VII IN THE REGION OF THE P-N JUNCTION.